1. Field of the Invention
The present invention relates to lens meters, and more particularly to an apparatus for measuring the interpupilary distance of spectacles mounted to a lens meter.
2. Description of Related Art
Conventionally, there are lens meters which measure the refractive power, axis, prism power and other characteristics of a spectacle lens, as shown in, for example, Japanese Patent Application Laid-open No. 60-17335. There is also provided an apparatus having an additional function of measuring the distance between optical centers of both of the lenses for the right and left eyes, in which the lenses are mounted on a spectacle frame (both eyes, pupilary distance (P.D.)) and the distance from a nose bridge center to an optical center of a right eye lens or a left eye lens (single eye's pupilary distance (P.D.)). In this specification, the interpupilary distance (P.D.) includes the binocular P.D. and the monocular P.D.
Another apparatus is disclosed in U.S. Pat. No. 4,098,002 to Campbell et al.. As shown in FIG. 1 of Campbell et al., at least one pad simulating the shape and location of the human nose, contacts the nose bridge of spectacles, slides on a bar which extends transversely of an optical path, whereby an optical center of a lens is matched to a measurement optical axis of the optical path. From that moving position of the pad, the monocular P.D. is measured. The apparatus has two pads for both of the lenses for right eye and left eye respectively. By calculating the right eye's P.D. plus the left eye's P.D., binocular P.D. is measured.
According to the above apparatus, there is an advantage of being able to measure the interpupilary distance when measuring the refractive power of a lens at the same time. However, there are following problems in this apparatus. First, the pad interferes with parts of lens fixing member. To remedy this, it is necessary to provide a pair of moving pads and a mechanism to detect their position. Therefore, the apparatus becomes very complex in structure and is very expensive.
Second, measuring only the refractive power of a lens mounted on a spectacle frame, similar to measuring the P.D., involves a different operation from the usual measurement of a lens meter. That is, after measuring one lens it is necessary to remove the spectacles from the pad and mount them on another pad. Even though the pad had been moved to the side position where it does not interfere with the parts by modification, the lens table becomes very large in size.